Lenovo ThinkPads work very nicely with Linux, and there’s a large second-hand market. The T and X series are especially great I find.
The usual advice about avoiding soldered RAM holds in general, but right now used laptops are being bought just for stripping RAM. So I think putting up with soldered RAM in second hand devices (I’d go for at least 32G) can be a smart move because it may be a better deal (and often a smaller form factor).





I have no experience with HPC clusters, just dropping in to contribute my opinions on the NixOS stuff.
This sounds good in theory, but in practice few people will have the know-how or patience to make changes to your config.
NixOS is very flexible and you can certainly bake custom compiled software into the builds, but expect a steep learning curve.